The radical species generated on hydroxyapatite (HAp) samples by UV irradiation were investigated and compared by electron spin resonance (ESR) with spin trapping reagent. It was recognized that ·OH and O 2 ·− species were produced on HAp after heat treatment at 200 °C (HAp200) by UV irradiation. It was assumed that ·OH radical was generated from H 2O on HAp200 by photo-induced excitation with UV irradiation as well as O 2 ·− from O 2 in air. These radicals were very active. On the other hand, few radical species were produced on HAp after heat treatment at 1150 °C by UV irradiation. The difference of generation of active radical species on HAp by UV irradiation must influence the activity for the photocatalytic decomposition of organic compounds.